The present invention principally relates to a noise reduction circuit for video signal processing for television receivers, VTRs, and the like. More particularly, the present invention relates to a noise reducing circuit suitable for use in noise reduction of the color difference signal and the luminance signal without bringing about an output voltage offset depending on the noise-reduction quantity and temperature variation and not producing a variation in the noise-reduction quantity.
As a conventional example of a noise reducing circuit, an arrangement disclosed in Japanese Laid-open Utility Model Publication No. 58-149832 is commonly known and a basic circuit diagram thereof is shown in FIG. 1. FIG. 2 is a diagram explaining the dependency on the noise-reduction quantity of the output voltage of the circuit of FIG. 1, and FIG. 3 is a diagram explaining the dependency on the ambient temperature of the noise-reduction quantity in the circuit of FIG. 1.
The conventional noise reducing circuit is composed, as shown in FIG. 1, of a push-pull type emitter-follower circuit formed of an NPN transistor 38 and a PNP transistor 39 with their emitters connected with each other, resistors 5 and 6 connected in series between bases of both transistors, current sources 4 and 7 connected to the resistors at their ends on the sides of the bases of both transistors for supplying current to the resistors, a capacitor 17 connected with the output point B of the push-pull type emitter-follower circuit, and a resistor 8 provided as a current path for charging and discharging the capacitor 17 from the input point A located at the junction of the resistors 5 and 6. In FIG. 1, an NPN transistor 2 and a resistor 3 form an input emitter-follower circuit and a PNP transistor 18 and a resistor 19 form an output emitter-follower circuit.
When V.sub.BE of the transistors 38, 39 are represented by V.sub.BE 38, V.sub.BE 39, the resistance values of the resistors 5, 6 by R.sub.5, R.sub.6, and the current flows from the current sources 4, 7 by I.sub.1, I.sub.2, respectively, the threshold level at which the transistor 38 on the .sym. side is turned on is expressed as (V.sub.BE 38-R.sub.5 I.sub.1) and the threshold level at which the transistor 39 on (the .crclbar. side is turned on is expressed as -(V.sub.BE 39-R.sub.6 I.sub.2), for a signal with a frequency sufficiently higher than the cutoff frequency of a low-pass filter formed of the resistor 8 and the capacitor 17. The noise-reduction quantity is then expressed as the sum total of such levels, i.e., EQU (V.sub.BE 38-R.sub.5 I.sub.1)+(V.sub.BE 39-R.sub.6 I.sub.2)
where
(V.sub.BE 38.gtoreq.R.sub.5 I.sub.1), PA1 (V.sub.BE 39.gtoreq.R.sub.6 I.sub.2).
When a rising signal with noise superposed thereon is input to the circuit of FIG. 1, the transistor 38 is turned on only when the noise level exceeds the .sym. threshold level and, during the off-period thereof, the waveform becomes a low-pass signal waveform by means of the resistor 8 and the capacitor 17. Accordingly, there is provided noise reduction at both rising and falling portions of the signal. In the conventional system, since the push-pull circuit is formed of an NPN transistor and a PNP transistor, the threshold level on the .sym. side and the threshold level on the .crclbar. side do not become completely symmetrical. On account of this asymmetry between these transistors, the output DC voltage at the point B varies as shown in FIG. 2. Therefore, when such conventional circuit is used as a noise reducing circuit in a television receiver for a color difference signal having high sensitivity to a DC voltage variation, there arises such a problem that the white balance of the television receiver suffers a change depending on the noise-reduction quantity and turning on/off of the circuit. This also leads to a problem of a variation of the black level with the luminance signal. Further, since, as described above, the noise-reduction quantity depends on the voltage V.sub.BE of the transistor, there has also been such a problem that the noise-reduction quantity varies with the ambient temperature as shown in FIG. 3.
In the above described prior art, no consideration has been given to the output DC voltage variation or offset depending on the noise-reduction quantity. Therefore, when such a system is used for a noise reducing circuit for a video signal, there arise such problems as production of a variation of the while balance and a variation of the black level, and hence degradation of the image quality of the set. Further, compensation for the temperature characteristic has been insufficient.